The radio transmitter of the mobile telephone in mobile telephony has a compact and space saving construction. The signal information (data, speech, control signals) to be transmitted are modulated on a carrier wave having a given angular frequency w.sub.c. The modulation method applied is so-called quadrature modulation, i.e. the carrier wave is divided into two quadrature components, sine w.sub.c t and cos w.sub.c t. These two components are then modulated with sine and cosine components of the information signal phase by, e.g., phase shift keying (QPSK). The information signal consists in a digital signal in the form of a bit flow of "ones" and "zeros". In the case of QPSK, a binary "one" corresponds to a given positive phase change or shift and a "zero" corresponds to a negative phase change or shift in the transmitted radio signal. The phase changes always start from the phase position of the preceding bit, so that subsequent to filtration the phase of the transmitted radio signal will have continuous progress in the absence of abrupt changes.
In order to form the radio signal r(t) to be transmitted, it is therefore necessary to form the sine and cosine values of a given phase angle (=phase change), these values being projected onto the two carrier wave components in the modulation process. These two values are called quadrature components and are normally designated I and Q respectively. It is known to use waveform generators which comprise memory stores in which these components are formed for a given phase change. For instance, U.S. Pat. No. 4,229,821 describes a waveform generator which contains two table look-up memories for each sine .phi. and cos .phi.. These two memories are addressed by a signal vector .phi. with a given number of bits depending on the duration of the low-pass-filter impulse response (the impulse response of the premodulation filters) included in the tables. The duration of the impulse response is normally truncated to a given number of bits, this number depending on the required quality of the transmitted radio signal.
Downstream of the modulator circuits in the transmitter is a final amplifier which operates in amplifying the radio signal r(t,.alpha.) to a given power for transmission from the transmitter antenna of the mobile telephone. Since the quadrature modulation applied is linear, i.e. the transmitted information influences both the amplitude and phase position of the radio signal, the amplification of the final amplifier must also be linear both with respect to amplitude and phase position.